The present invention relates to a signal processing apparatus for use with a single chip solid-state color camera, and more particularly, to a signal processing apparatus which allows achievement of both noise reduction and chroma signal demodulation.
Single chip solid-state color cameras using a solid-state imaging device such as a two-dimensional charge transfer device (CTD) (e.g., a charge coupled device (CCD), a charge sweep device (CSD) and the like) have been used majorly in home video cameras because of high reliability, productivity and so on.
A CCD solid-state imaging device comprises a plurality of photoelectric conversion elements arranged, for example, in rows and columns, a color filter including plural sorts of color filter elements which are mutually different in their transmission lights and are disposed to respectively cyclically correspond to the plurality of photoelectric conversion elements, a charge transfer means for transferring pixel signal pieces of the plurality of photoelectric conversion elements in the row direction, and an output means for extracting the pixel signal pieces of the photoelectric conversion elements transferred by the charge transfer means sequentially with a fixed period or at constant intervals.
An example of techniques for the purpose of reducing noise in the output of the single chip solid state-color camera employing the aforementioned CCD solid-state imaging device has been disclosed in, for example, NO. JP-A-60-10500 (laid open on Jan. 1, 1985) wherein the intended purpose is realized by the use of a circuit having such an arrangement as shown in FIG. 1.
In FIG. 1, an output signal of a CCD imaging device 1 is supplied to a video signal processing circuit 5 through a preamplifier 2, a DC restoration circuit 3 and a sample-hold circuit 4.
A driving pulse signal generated by a pulse generator 6 is first supplied to a driving circuit 7 to be shaped therein and then is applied to the CCD imaging device 1.
As a result, the output signal of the imaging device 1 has such a waveform as shown in FIG. 2(a).
In this connection, a reset pulse signal applied to the imaging device 1 may have such a waveform as shown in FIG. 2(b), while the driving pulse signal applied to a driving electrode located nearest to a signal detector part 1S, which forms output means of the imaging device, has such a waveform as shown in FIG. 2(c).
In the signal waveform shown in FIG. 2(a), a duration from time t.sub.0 to time t.sub.1 is a reset duration during which a reset pulse signal (b) is used to clear the pixel charges accumulated in the signal detector part 1S. During this charge clearing operation, noise generated by a switching transistor Tr cyclically on-off controlled to send a pixel signal remains in the signal detector part 1S as reset noise and lasts to the starting end of the next reset duration. The photodiode charges or pixel charges are carried to the signal detector part 1S by a driving pulse signal (c) applied during a time between time t.sub.2 and time t.sub.3.
Consequently, in a duration from time t.sub.3 to time t.sub.4 from the falling edge of the reset pulse signal (c) to the beginning of the next driving operation, an image signal including pixel signal pieces superposed with reset noise is generated from the imaging device.
As will be appreciated from the foregoing explanation, in the pixel signal waveform of FIG. 2(a), only the reset noise appears in a duration from time t.sub.1 to time t.sub.3 while the pixel signal piece superposed with the reset noise appears in a duration from time t.sub.3 to time t.sub.4. Therefore, when such a timing pulse signal as shown in FIG. 2(d) is applied from a pulse generator 6 to the DC restoration circuit 3 in FIG. 1 so that the DC level of an output signal of the circuit 3 is clamped to a constant level during a time interval from the rising edge of the timing pulse (d) to the beginning of the reset period of the reset pulse signal (b) within the t.sub.1 -t.sub.4 duration, there can be obtained from the circuit 3 a noise disabling output signal which has such a waveform as shown in FIG. 2(e).
When the noise disabling signal is applied to the sample-hold circuit 4 to sample it within the t.sub.3 -t.sub.4 time duration by the use of such a timing pulse signal sent from the pulse generator 6 as shown in FIG. 2(f), the circuit 4 can produce such a pixel signal with the reset noise removed as shown in FIG. 2(g).
Meanwhile an example of a single chip solid-state color camera employing a CCD imaging device is disclosed in, for example, No. JP-A-60-3290 (laid open on Jan. 9, 1985) wherein an imaging device 1' comprises such a color filter made up of elements arranged in a mosaic or matrix form as shown in FIG. 3 and an image signal from the imaging device 1' is processed by such a signal processing circuit as shown in FIG. 4.
The schematic operation of the prior art example is as follows.
That is, with the color filter of the matrix shown in FIG. 3, the color filter elements W (allowing passage of all the color light components therethrough), Cy (allowing passage of cyan light component therethrough), W and Ye (allowing passage of yellow color light component therethrough) are cyclically arranged in the first column H.sub.1 ; and the color filter elements G (allowing passage of green color light component therethrough), Ye, G and Cy are cyclically arranged in the second column H.sub.2.
Color filter elements, cyclically arranged as described above are provided for respective pixels or photodiodes in the imaging device. When pixel signal pieces are read for every two adjacent lines of the CCD imaging device provided with the color filter and mixed with each other (the so-called field storage mode interlace), the signal detector part or the output means of the imaging device delivers, for example, a mixture signal of W (H.sub.1, V.sub.1) and Cy (H.sub.1, V.sub.2) and a mixture signal of G (H.sub.2, V.sub.1) and Ye (H.sub.2, V.sub.2) sequentially in the first line scanning operation and a mixture signal of W (H.sub.1, V.sub.3) and Ye (H.sub.1, V.sub.4) and a mixture signal of G (H.sub.2, V.sub.3) and Cy (H.sub.2, V.sub.4) in the second scanning line operation of the first field.
The output signals obtained through the first and second line scanning operations have such components as shown in FIGS. 5(a) and 5(b), respectively. More specifically, a frequency modulated chroma signal B appears in the output signal of the first scanning operation as, while a chroma signal R appears in the output signal of the second scanning operation.
In the second field where the combination of two lines to be mixed is shifted by one line from that in the first field, a mixture signal of Cy (H.sub.1, V.sub.2) and W (H.sub.1, V.sub.3) and a mixture signal of Ye (H.sub.2, V.sub.2) and G (H.sub.2, V.sub.3) are sequentially obtained in the first scanning operation, while a mixture signal of Ye (H.sub.1, V.sub.4) and W (H.sub.1, V.sub.5) and a mixture signal of Cy (H.sub.2, V.sub.4) and G (H.sub.2, V.sub.5) are sequentially obtained. Accordingly, the chroma signals B and R subjected to a frequency modulation similarly appear in the respective output signals.
In the prior art signal processing circuit of such an arrangement as shown in FIG. 4, an output signal of the CCD imaging device 1' is applied through the preamplifier 2 to a low pass filter 8 to obtain a luminance signal and also to a band pass filter 9.
The band pass filter 9 outputs alternately the frequency-modulated chroma signals B and R alone at intervals of one horizontal scanning period. The chroma signals B and R are applied to a detector circuit 10 to be demodulated to respective base band signals. The base band signals are supplied from the detector 10 to a 1H line delay circuit 11 so that the chroma signals B and R are simultaneously applied to an encoder circuit 12 together with the luminance signal sent from the low pass filter 8 to obtain a color video signal from the circuit 12. A pulse generator 6 and a driving circuit 7 may be similar ones in FIG. 1.
When the two prior art signal processing circuits of FIGS. 1 and 4 are combined together, there can be realized such a single chip solid-state color camera that uses a CCD imaging device and produces less noise. For example, refer to a paper entitled "Color Characteristics of 1/2-Inch PAL system CCD" reported by Ohmae et. al. in Preliminary Transactions for 1985 Nation-wide Congress of The Institute of Television Engineers of Japan, 1985, pp. 83-84. The combined circuit arrangement may be as shown in FIG. 6. That is, the combination circuit requires the DC restoration circuit 3 and the sample-hold circuit 4 for noise suppression and requires the band pass filter 9 and the detector circuit 10 for demodulation of the chroma signals in the course from generation of the output signal of the CCD imaging device 1' until demodulation of the chroma signals while suppressing reset noise. In this way, the combination circuit of FIG. 6 is increased in the circuit structure scale as compared with that of the prior art of FIG. 4 without noise reduction function.
Further, with respect to a color filter to be used in the single chip solid-state color camera, it is difficult for the respective color filter ,elements to pass 100% of their transmission lights (that is, the filters having respectively a transmittance of 100%) and it is also difficult to make equal peak values in the transmittance characteristics of the color filter elements allowing passage of their different transmission color light components.
For example, with the color filter shown in FIG. 3, if the filter elements W for passage of whole color light components have all a transmittance of 100% because they have no filtering action and the filter elements Cy, Ye and G have respectively a transmittance of 90%, then the following mixture or added signals can be obtained during each horizontal scanning interval. EQU W+Cy=1.9G+1.0R+1.9B (1) EQU Ye+G=1.8G+0.9R (2) EQU W+Ye=1.9G+1.9R+1.0B (3) EQU Cy+G=1.8G+0.9B (4)
The chroma signal obtained by passing the modulated components (W+Cy) and (Ye+G) through the band pass filter corresponds to a difference between the equations (1) and (2). That is, the chroma signal is expressed by the following equation (5). ##EQU1##
Since the chroma signal obtained from the modulated components (W+Ye) and (Cy+G) corresponds to a difference between the equations (3) and (4), the signal is expressed as follows. ##EQU2##
It will be seen from equations (5) and (6) that when the color filters have such transmittances as mentioned above, the chroma signals and obtained according to the conventional demodulation system contains color mixture components ##EQU3## respectively.
As has been explained in the foregoing, the single chip solid-state color camera using the conventional noise reduction method has such a problem that the camera is increased in the circuit structure scale when compared with the camera not having a noise reduction function, whereas the single chip solid-state color camera using the conventional color signal demodulation method has such a problem that no consideration is paid to variations in the peak values of the transmittance characteristics of the color filter elements and the demodulated chroma signal contains the color mixture component in this camera.